A small discharge lamp which is denoted by a metalhalide lamp or an ultra high pressure mercury vapor lamp is widely utilized as a light source for a projection display apparatus and the like. In such a case, it is general to combine the discharge lamp with a concave reflector to compose a light source apparatus and utilize this apparatus as a light source for the projection display apparatus.
FIG. 17 exemplifies a configuration of a conventional discharge lamp. A discharge lamp 321 is configured mainly by a light emitting bulb 301, sealing members 302 and 303, metal foils 304 and 305, electrodes 306 and 307, external conductors 308 and 309, and discharge media 310, 311 and 312. Quartz glass is used as the light emitting bulb 301 and sealing members 302, 303, tungsten is used as the electrodes 306 and 307, molybdenum foils are used as the metal foils 304 and 305, and molybdenum is used as the external conductors 308 and 309. Furthermore, mercury, a light emitting metals such as a metalhalide or the like, and a rare gas such as argon or the like, are used mainly as the discharge media 310, 311 and 312, respectively.
When a predetermined voltage is applied across the external conductors 308 and 309, arc discharge takes place between the electrodes 306 and 307, whereby the mercury 310 and the metal halide 311 emit rays characteristic thereof. The argon gas 312 is used to improve a starting characteristic.
Since a distance is extremely short between the electrodes and a high current is supplied at a start time in this kind of discharge lamp, the lamp-is liable to be blackened due to deformation of the electrodes and evaporation of an electrode substance, and can hardly have a long service life. In contrast, there have been disclosed various kinds of lamps which are configured to have service lives prolonged by contriving structures of electrodes (for example by JPA 7-192688 and JPA 10-92377). FIGS. 18 through 20 are enlarged views exemplifying configurations of the electrodes.
FIG. 18 shows an example wherein a coil 331 is disposed around a tip of an electrode 330 to enhance a heat dissipation property, thereby preventing a tip portion from being deteriorated or deformed due to excessive temperature rise.
FIG. 19 shows an example wherein a discharge portion 342 which has a diameter larger than that of an electrode shaft 341 is formed at a tip of an electrode 340 to enhance a thermal conductivity, thereby preventing a tip portion from being deteriorated or deformed due to excessive temperature rise. This kind of electrode is used as an anode of a DC type discharge lamp.
FIG. 20 shows an example wherein a discharge member 352 having a diameter larger than that of an electrode shaft 351 is formed by winding a coil thick around a tip of an electrode 350 and fusing a tip portion so as to form a lump integral with an electrode shaft 351, and a heat dissipating member 353 is formed after the discharge member 352 by integrally fusing a coil, thereby preventing the electrode from being deteriorated or deformed. The heat dissipating member 353 is configured by a coil or a cylindrical electrode member.
However, the electrodes which have configurations shown in FIGS. 18 through 20 pose problems which are described below.
In case of the configuration shown in FIG. 18, a contact area between the electrode 330 and the coil 331 is narrow, whereby the electrode has a low thermal conductivity and cannot exhibit a sufficient heat dissipating effect. Furthermore, the electrode poses a problem that the coil 331 is fused and deformed when the coil 331 is too thin. Though this problem can be solved by thickening the coil 331, tungsten which is used as a material of the electrode 330 is hard and the coil 331 can hardly be wound when it is thick. Furthermore, the electrode poses another problem that a spot of arc discharge moves to the tip of the electrode or an end of the coil, whereby an arc is hardly be stable.
In case of the configuration shown in FIG. 19, the discharge member 342 which is too thick makes the electrode 340 hardly be heated to a temperature required to emit thermoelectrons, thereby posing a problem of degradation of a starting property and interception of discharge. This is remarkably problematic when a lamp is to be lit with an alternating current in particular, whereby the electrode can hardly be used for lighting a lamp with an alternating current.
In case of the configuration shown in FIG. 20 wherein the discharge member 352 is formed integrally and continuously with the coil 353, the discharge portion 352 and the coil 353 have high thermal conductivities and are hardly be raised to a temperature required to emit thermoelectrons, thereby degrading a starting property or allows discharge to be intercepted in the course like the structure shown in FIG. 19. This poses a serious problem when a discharge lamp is to be ignited with an alternating current in particular. Furthermore, an electrode such as that shown in FIG. 20 is manufactured by allowing the electrode having the coil 353 wound around the electrode shaft 351 to discharge in an atmosphere of an inert gas such as nitrogen gas or argon so as to fuse the tip portion. A doping agent such as thorium is often added to tungsten as electrode material for a discharge lamp to improve a starting property. However, the electrode manufactured by the method described above poses a problem that the doping material is evaporated at a stage to fuse the tip portion. Furthermore, the electrode poses another problem that the fusing promotes recrystallization of the tip portion, whereby the electrode is low in its strength and can hardly be worked.
When this kind of discharge lamp is to be used in a projection display apparatus, on the other hand, it is general to configure a light source by combining the discharge lamp with a concave reflector. FIG. 21a exemplifies a configuration of a light source. FIG. 21b is a sectional view taken along an A—A line in FIG. 21a. A reflective coating 372 which is formed on an inside surface of a concave reflector 371 reflects rays emitted from a lamp 360 in a predetermined direction with a high efficiency. A lamp insertion port 373 and a conductor outlet port 374 are formed in the concave reflector 371. The lamp 360 is fixed to the concave reflector 371 with a heat-resistant adhesive agent 375 after inserting a sealing member 362 is inserted into the lamp insertion port 373. Furthermore, an end of an extension conductor 376 is connected to an external conductor 369 and the other end of the extension conductor 376 is led out of the concave reflector 371 through the conductor outlet port 374. Rays can be emitted from the lamp 360 by applying a predetermined voltage across an external conductor 368 and the extension conductor 376.
It is desired that a lamp which is to be used in the projector display apparatus is as small as possible and has a long service life. However, the conventional light source shown in FIG. 21a poses problems which are described below.
First, the conventional light source poses a problem that oxidation of metal foils 364 and 365 disposed at both ends of the lamp 360 as well as the external conductors 368 and 369 results in wire breakage, thereby shortening a service life of the lamp. In case of the light source shown in FIG. 21a, distortion is produced by a thermal stress at a sealing stage, whereby a gap B is formed between the external conductor 369 and a sealing member 363 as illustrated in FIG. 21b showing an enlarged sectional view taken along the A—A line. Accordingly, the external conductor 369 and an end of the metal foil 365 on a side of the external conductor 369 are kept in contact with air, whereby oxidation of these parts is accelerated in an extremely high temperature condition while the lamp stays lit. When molybdenum is used as the metal foils, for example, the oxidation results in wire breakage in a time of about 5000 hours in air heated to 350° C. though the time is variable dependently on a temperature. The external conductor 368 and the sealing member 362 are also oxidized in the similar manner.
While the discharge lamp used in the projection display apparatus stays lit, the lamp is generally kept at an extremely high temperature and heats a light emitting bulb 361 to a temperature close to 1000° C. at maximum. Accordingly, temperatures reach hundreds of degrees in the vicinities of connected portions between the metal foils 364, 365 and the external conductors 368, 369 due to heat conduction from the light emitting bulb 361 as well as electrodes 366 and 367. Though the temperatures can be lowered by forcible air cooling with a fan or the like, evaporation of the light emitting metal is suppressed and a light emitting efficiency is remarkably lowered when the temperature of the light emitting bulb 361 is lowered. Therefore, it is therefore required to cool the lamp extremely locally with high delicacy.
In order to solve this problem, the conventional discharge lamp uses sufficiently long metal foils, thereby reducing temperature rise due to the heat conduction and preventing the wire breakage due to the oxidation. However, the conventional discharge lamp has a total length which is prolonged by the long metal foils and poses a problem that the lamp makes it difficult to configure a light source compact.
Secondly, the conventional light source poses another problem that evaporation of the light emitting metal which is evaporated while the lamp stays lit enhances an internal pressure of the light emitting bulb to an extremely high level, for example, of several MPas (mega pascals) in case of the metalhalide lamp or of scores of MPas (mega pascals) in case of the super-high pressure mercury lamp, thereby making the light emitting bulb liable to be broken while the lamp stays lit.